Views: 90 Author: Site Editor Publish Time: 2021-09-10 Origin: Site
Fluorescent powderis fluorescent because it contains fluorescent dyes. The following explains the changes that the fluorescent dye molecule undergoes during fluorescence.
Molecules are also composed of a nucleus and electrons, which have energy and are roughly divided into nucleus-to-electron attraction, electron-electron repulsion, and electron-related energy. The first two are classical Coulomb interactions and the last one comes from quantum effects. The combination of these results in a series of discrete values for the energy of the electron in a molecule (this is also a quantum effect), and we usually call the possible values of these energies the electron energy levels.
An electron with a certain energy has a certain corresponding state of motion, usually called a molecular orbital. After we use a series of approximations, we can consider that two electrons can be placed on each molecular orbital, which have opposite spins. The number of electrons within a molecule is finite, but the number of molecular energy levels (and corresponding molecular orbitals) is infinite.
Molecular stability requires the lowest total energy of the entire molecular system, so the electrons will start from the lowest energy orbital and N electrons will occupy N/2 energy levels, which is the electronic configuration of the ground state. The spacing of the electron energy levels is typically a few electron volts (eV).
In addition to the electronic state, there is also the vibration of the molecule that affects the energy of an isolated molecule, which is actually a small movement of the nuclei around the equilibrium position. This motion is manifested by changes in bond lengths, bond angles, dihedral angles, and more complex geometrical factors, typically on the order of 0.0025 to 0.5 eV. The electronic and vibrational energy levels can usually be treated separately, and the following Jablonski diagram can be obtained, where only two electronic states, S0 and S1, are listed. Each electronic state can be considered as a manifold of dynamics.
Now let's look at the absorption of photons. Initially, the molecule is in the ground state, which is the 0 of S0. After a photon of suitable energy hits it, the electron absorbs the energy of this photon and jumps to the third vibrational excited state of the first electronic excited state. This process is energy conservation, the molecular system energy increased \hbar\omega, \hbar is Planck's constant divided by 2\pi, \omega is the photon angular frequency.
Of course, if the photon energy is slightly more or less, the electron will jump to some state higher or lower, if this state exists. After the absorption is complete, the molecule is in a less stable state and has a tendency to release energy back to the ground state.
There are various ways to release energy, such as the red arrow in the diagram, which is the relaxation of vibrational energy level, through the process of collision between molecules to turn the vibrational energy into molecular kinetic energy, and macroscopically the system temperature increases, that is, it becomes heat.
After relaxing in some way to the vibrational ground state of the first excited state, an electronic leap occurs and returns to the electronic ground state. The final state of this leap is also possible in many ways and can return to any vibrational excited state of the electronic ground state. During this process, the system emits photons with reduced energy, which is fluorescence.
There is a time difference between absorption and emission, which can be considered as the lifetime of the electronic excited state. The lifetime of the excited state varies greatly from system to system. For example, some heme proteins have a lifetime of several tens of femtoseconds (1E-15s), while the first excited state of phenol has a lifetime of several nanoseconds (1E-9s). After excitation, the fluorescence intensity decays exponentially with time.